Acid hydrolysis of biomass and cellulose is known in the art. It is practiced both in homogenous or heterogeneous manners.
Usually processes to obtain a monomeric sugar stream from an oligomeric sugar stream coming from pre-treated lignocellulosic material (e.g. autohydrolysis, hot water washing or steam explosion) seek to limit the monosaccharide transformation into degradation products during post hydrolysis of the oligosaccharides into monomers. (Duarte et al., 2004; Girio et al., 2010).
Post hydrolysis, also known as hydrolysis, options for the xylo-oligo-saccharides (XOs) hydrolysis are acid catalyzed (Boussaid et al., 2001), or enzymatic catalysed processes (Duarte et al., 2004) (Carvalheiro et al., 2008).
The main factors affecting monosaccharide recovery in dilute-acid chemical post hydrolysis are catalyst concentration, reaction time, and temperature. The acid process was applied to hydrolysates obtained from softwoods (Shevchenko et al., 2000), hardwoods (Garrote et al., 2001a) and herbaceous materials (Garrote et al., 2001b). The main catalyst reported is sulphuric acid (Duarte et al., 2009; Shevchenko et al., 2000), although, other catalysts can be employed (such as phosphoric acid, hydrochloric acid, formic acid). Under fully optimized post hydrolysis conditions, sugar recovery can be close to 100% (Duarte et al., 2004, 2009; Garrote et al., 2001a,b; Shevchenko et al., 2000), as compared to the standard dilute acid hydrolysis (121° C., 4% H2SO4 and 60 min) which is generally used for the quantitative acid hydrolysis of oligosaccharides. During the acid hydrolysis of oligosaccharides, degradation reactions lead to the formations of many compounds, particularly, 5-hydroxymethylfurfural (HMF), furfural, formic and levulinic acids, which can inhibit further bioconversion steps, reducing the sugar yields of the process (Duarte et al., 2009).
Additionally, acid catalysts usually involve increasing the concentration of non-sugar compounds up to a value incompatible with the economic and environmental sustainability.
Kim et al (Youngmi Kim, Rick Hendrickson, Nathan Mosier, and Michael R. Ladisch, “Plug-Flow Reactor for Continuous Hydrolysis of Glucans and Xylans from Pretreated Corn Fiber”, Energy & Fuels 2005, 19, 2189-2200), describes a heterogeneous system when the aqueous stream is first contacted with the cation exchanger at room temperature where proteins, phenolics, minerals, and other catalyst fouling components are removed. The material is then passed over a packed-bed of the same catalyst at 130° C. to give 88% hydrolysis for a space time of 105 min.
The process in Kim et al is temperature limited because the catalyst degrades at temperatures greater than 130° C. and catalyst fouling also increases with increasing temperature above than 130° C.
Alternatively, the post hydrolysis of oligosaccharides can be catalysed by enzymes. Because the complex hemicellulose structure is still present in the oligosaccharides obtained from the pre-treatment, the action of several enzyme activities is usually required for the complete hydrolysis (e.g., endoxylanase, exoxylanase, β-xylosidase and accessory activities like acetyl xylanesterase, α-glucuronidase, α-arabinofuranosidase, and feruloyl esterase); hence potentially turning the process uneconomical (Vázquez et al., 2002; Duarte et al., 2009). Moreover, toxic/inhibitors compounds potentially present in the hydrolysate can significantly reduce enzyme activity (Carvalheiro et al., 2008). Regardless of all theses aspects, the enzymatic posthydrolysis presents the advantage of minimizing the monosaccharide degradation reactions.
There exists therefore, a need for a homogeneous acidic catalyzed hydrolysis which produces few degradation products.